1. Field of Invention
The present invention relates to a vehicle height adjust control apparatus and method for setting a vehicle height to a target vehicle height by supplying hydraulic fluid to and discharging it from hydraulic actuators provided between a vehicle body and wheels.
2. Description of Related Art
A vehicle height adjust control apparatus described in Japanese Patent Application Laid-Open No. Hei 2-3515, for example, includes a hydraulic actuator for changing a vehicle height by hydraulic fluid supplied and discharged, a supply-discharge device made up of a hydraulic pump, a control valve for supplying hydraulic fluid into and discharging it from the hydraulic actuator, a vehicle height detection device for detecting an actual vehicle height, and a supply-discharge control device for controlling the operation of the supply-discharge device so as to eliminate deviation of the detected actual vehicle height from a predetermined target vehicle height. The apparatus controls and sets the actual vehicle height to a target vehicle height by supplying hydraulic fluid to and discharging it from the hydraulic actuator using the supply-discharge device.
In this vehicle height adjust control apparatus, various problems occur depending on various factors, such as the construction of the hydraulic pump or the control valve, the control of the supply-discharge device by the supply-discharge control device, the condition of the hydraulic fluid, the condition of a battery, and the like. Specifically, if the hydraulic pump is operated at a very low hydraulic fluid temperature, an excessively great load is caused on the hydraulic pump. This adversely affects the durability of the hydraulic pump because at a very low temperature the fluidity of hydraulic fluid is very low and the viscosity thereof is very high. At a very high hydraulic fluid temperature, the viscosity of hydraulic fluid becomes very low, so that the ejecting performance of the hydraulic pump decreases. Therefore, it becomes necessary to operate the hydraulic pump for longer periods. Long-time operation of the hydraulic pump further increases the hydraulic fluid temperature, thereby adversely affecting the durability of the hydraulic pump. This temperature-dependent problem is significant, particularly if the hydraulic pump is a gear pump.
If stopping of the hydraulic pump and switching of the control valve from a open state to an close state are simultaneously performed to stop the supply of hydraulic fluid from the hydraulic pump to the hydraulic actuator, hydraulic fluid may impact the control valve because the hydraulic pump will not immediately stop ejecting hydraulic fluid, due to the inertia of an electrical motor or the like. Such impact on the control valve produces impact noise and degrades the durability of the hydraulic system, including the hydraulic pump, the control valve and the like. This problem becomes more significant if the hydraulic fluid temperature is lower and the hydraulic fluid viscosity is higher, and if the hydraulic fluid ejecting pressure produced by the hydraulic pump is higher.
If the control valve is an electromagnetic on-off valve, a size reduction of the control valve results in a reduction in the number of turns of the coil, so that it may become difficult for the coil to provide a sufficiently great attraction force for drawing a plunger. Furthermore, since a temperature increase correspondingly increases the resistance of the coil, the coil may not be provided with a current sufficient to attract the plunger if the hydraulic fluid temperature is high. In addition, an insufficient current through the coil for attracting the plunger also results from a voltage reduction of the battery that energizes the coil.